The invention relates to a composition which is effective in: increasing the cuttings carrying capacity of a clay-containing drilling fluid; formulating spacer fluids for use in cementing operations in oil and gas wells; and for formulating lost circulation pills for use in well servicing applications.
It is generally agreed among those skilled in the art that a rotary system is an acceptable form of drilling an oil or gas well. This system depends upon the rotation of a string of drill pipe to the bottom of which is attached a multi-pronged drilling bit. The bit cuts into the earth causing the cuttings to accumulate as drilling continues. As a result, a fluid must be used to carry these cuttings to the surface for removal, thus allowing the bit to continue functioning and the bottom hole to be kept clean and free of cuttings at all times. Drilling systems other than the rotary system are sometimes employed during drilling operations. Nevertheless, these systems still require a drilling fluid to remove the borehole cuttings and to otherwise perform functions related to drilling fluids.
During the initial stages of drilling an oil and gas well, a large diameter borehole is desired. This necessitates the removal of a large volume of drill cuttings from the borehole, and thus the drilling fluid must have excellent carrying capacity as exhibited by a high yield point, and particularly a high yield point to plastic viscosity ratio.
The characteristics of the mud used for spudding-in a well vary widely over the world. Some locations are spudded-in through conductor pipe already cemented in the collar, and some locations require that the length of the kelly and a few joints of drill pipe be drilled down, in order to cement one or more joints of conductor casing. With the latter wells, the circulation is frequently carried through a ditch dug in the surface soil. The cement drilled out of the conductor contaminates the spud mud. Most of the contaminated mud will be discarded before drilling continues below the conductor.
Frequently the formations near the surface are soil, loose sand, and gravel. The mud should form a wall cake on these formations to prevent caving and hole enlargement, and should have body enough to carry out the cuttings and gravel encountered in this fast drilling section. When the make-up water is soft and the formations near the surface make mud, an ideal situation exists for spudding in a well. When conditions are otherwise bentonite or clay is mixed to make spud mud, and sometimes the make-up water must be treated with soda ash and lime.
With fair make-up water, 20 sacks of bentonite will make from 80 to 100 bbl. of mud; 20 sacks of premium clay will make from 45 to 65 bbl. of mud; and 20 sacks of the cheaper drilling clays will make from 10 to 20 bbl. of mud. Annular velocities are low in the large conductor pipe hole, and many operators use enough clay to make a thick mud that will more readily remove surface sands and gravels. Generally, only bentonite or premium clay is used. Usually the spud mud will be mixed by one member of the crew while the final rigging up is being done. Water is run into the pit and circulated through the hopper where the clay is carefully mixed at a rate of from 2 to 15 minutes per sack. If any clay balls up and floats on the surface, it indicates that the mixing has probably been at too fast a rate, and the guns must be used to stir in the clay. The mixing of clay should be continued until a funnel viscosity of 30 to 35 seconds is reached. The viscosity will increase as the mud ages and picks up solids while drilling.
Many operators mix clay and water and then add lime. Lime thickens the mud and less clay is required to build any desired viscosity. The spud mud is usually discarded or watered after surface pipe is set; hence the addition of lime is not as objectionable as with muds used in deeper drilling, where good filtration properties are desired.
Circulation in a drilling well can be lost into fractures induced by excessive mud pressures, into pre-existing open fractures, or into large openings with structural strength. Depending on the cause of loss, remedial measures involve reducing the pressure exerted by the circulating fluid, or plugging the openings through which the drilling fluid is escaping. The openings may be closed by introducing a slurry which becomes stiff, or by introducing a bridging or plugging solid so that normal filtration can occur. Often the bridging or plugging solids are included in compositions that form stiff plugs or set hard. Many materials have been used to prevent or overcome lost circulation, including asbestos, bagasse, flax shives, hog hair, leather, mineral wool, paper, wood products, and cotton products.
Spacer fluids are used in well servicing operations to prevent the inter-mixing of two fluids, such as oil mud and water mud, drilling fluids and cement, drilling fluids and completion fluids, and the like. Spacer fluids are generally viscous and may be of high density depending on the density of the fluids which it is desired to keep separated.
It is an object of this invention to provide a well servicing fluid additive which is effective in enhancing the cuttings carrying capacity of an aqueous clay-containing drilling fluid.
It is another object of this invention to provide an additive which can be easily mixed with water to form a lost circulation pill or spacer fluid.
It is still another object of this invention to provide a method of alleviating lost circulation in a borehole and a method of preventing the admixing of two dissimilar fluids in a borehole.
While the invention is susceptible of various modifications and alternative forms, specific embodiments thereof will hereinafter be described in detail and shown by way of example. It should be understood, however, that it is not intended to limit the invention to the particular forms disclosed, but, on the contrary, the invention is to cover all modifications and alternatives falling within the spirit and scope of the invention as expressed in the appended claims.
The compositions can comprise, consist essentially of, or consist of the stated materials. The method can comprise, consist essentially of, or consist of the stated steps with the stated materials.